1. Field of the Invention
The field of the invention relates to the field of digital data handling and in particular to providing an intermediate voltage to enable devices designed to operate in one voltage domain to operate in a circuit powered in a higher voltage domain.
2. Description of the Prior Art
Advances in the field of electronics have led to a reduction in transistor dimensions and oxide thickness in order to increase processing speeds and reduce area. A corresponding reduction in the voltage powering both core and input-output devices in order to increase speed and reduce power consumption have also occurred.
Thus, for devices of 45 nm, the “standard” external power is now 1.8V where it was previously 3.3V or 2.5V. In order to be able to reach high frequencies the oxide thickness has been decreased and is now around 28 to 32 Å for 1.8V devices (where previously it was about 50 Å). Devices of lower sizes will operate at even lower voltages.
Apparatus that operate with these new smaller more fragile devices in circuits may have to handle signals suitable for the old higher voltage domains. For example, to be compatible with some older chips and a few standard protocols some input-output cells must operate at a nominal 3.3V in order to be able to generate signals at this level using the devices that operate at a nominal 1.8V.
In order to ensure that these devices are not overstressed with the accompanying problems of oxide breakdown and lifetime degradation due to HCI (hot carrier injection) precautions need to be taken.
One way of protecting the devices is to provide an intermediate voltage supply, such that the higher voltage domain is divided in two.
This intermediate voltage supply may be the high voltage supply of the lower power domain of the new devices, in this example 1.8V. The devices can then be arranged either between this intermediate voltage level and the high voltage level of the higher voltage domain, in this case 3.3 V, or between the intermediate voltage level and ground. In this way the devices only see 1.8V or 3.3 V-1.8 V that is 1.5V across them and they are protected.
An advantage of such a system is that there will already be a voltage supply supplying the 1.8V level and thus, no additional voltage level generator is required. Voltage level generators take both power and area. However, a disadvantage is that if the voltage level of the high voltage level domain varies, there are corresponding variations in the intermediate level voltage. Furthermore, devices operating between 3.3V and 1.8V have 1.5 V across them so that they will operate more slowly than those operating close to the optimal operation point of 1.8V.
An alternative solution would be to provide an intermediate voltage that is the high voltage level divided by two. This would mean that both halves of the circuit would have the same voltage difference across them. However, an additional voltage level generator that consumes power and area is required and again the voltage difference used to power the devices is quite far from the optimal value resulting in slower operating speeds.
It would be desirable to provide a flexible system that can handle higher voltage levels using devices that are designed for lower voltage levels and yet can maintain a voltage level close to the optimal operating voltage levels across the devices even where the input high voltage level varies.